1. Technical Field
This invention generally relates to semiconductor devices, and more specifically relates to biasing currents in semiconductor devices.
2. Background Art
Phase-locked loops (PLLs) are used a wide variety of applications in semiconductor devices. For example, PLLs are used in clock generators, frequency multipliers, frequency synthesizers and servo systems in disk drives. Naturally, in all of these and other applications the accuracy and reliability of the PLL is of critical importance.
Turning to FIG. 1, FIG. 1 is a schematic view of a prior-art phase-locked loop 100. PLL 100 comprises a phase comparator 102, a charge pump 104, a filter 106, a voltage-to-current converter 108, a current-controlled oscillator (ICO) 110, an output circuit 112, a feedback divider 114, and a bias current source 116.
The general operation of PLL's is well known, so only a brief explanation will be given. Phase comparator 102 compares an input signal to a feedback signal from feedback divider 114. Depending upon the phase difference between the input signal and feedback signal, the phase comparator drives charge pump 104. The output of charge pump 104 is filtered by filter 106, and is used to drive voltage-to-current converter 108. Voltage-to-current converter 108 outputs a current that is proportional to the voltage at its inputs. The output current from voltage-to-current converter 108 is used to drive ICO 110, thereby controlling the frequency of the output of ICO 110.
The output of ICO 110 is buffered by output circuit 112, converting the differential output signal to a single-ended output, which is then output and fed back through feedback divider 114 to phase comparator 102. This creams the feedback that facilitates the phase-locked loop operation.
Several bias currents are needed to provide stable and accurate operation of the PLL. In particular, a bias current is used on charge pump 104 to regulate the gain of the PLL, the gain being proportional to the charge pump bias current. In addition, a bias current is used by voltage-to-current converter 108 to set its operating point at the required value. Finally, a bias current is used by ICO 110 to keep it oscillating at a particular center frequency in the presence of voltage, temperature and process variations.
These various bias currents are provided by bias current source 116. Because of the tight tolerances in bias currents required, the prior art has used elaborate and complicated current reference circuits that require special components and careful design. For example, bias current source 116 is typically something like a band gap regulated current source. These current sources use diffused diodes. Fabricating these diodes requires additional process steps that are not normally used in the manufacture of CMOS integrated circuits and may create problems for both yield and tolerances as the process geometries shrink. Additionally, there are few other components available that could be used to supply the necessary bias currents, especially in CMOS technologies. Without these complicated circuits, the prior art approaches required that wide variations in bias current be tolerated at the expense of broad ranges of input control voltage that causes other design difficulties.
The input elements of ICO 110 are shown in an exploded portion 118. This input portion serves to control the oscillator circuit of ICO 110 (not shown), resulting in an output with a frequency proportional to the input current. The input elements include a current mirror comprising N-FETs 130 and 134. The output current of voltage-to-current converter 108 and the bias current from bias current source 116 drives into a diode created by N-FET 130. The current flowing through N-FET 130 is mirrored by N-FET 134. The current flowing through N-FET 134 serves as the input to the oscillator portion 118 of ICO 110.
Thus, PLLs employing current controlled oscillators require bias currents to sustain oscillation at particular desired frequencies. To achieve the required stability in such bias currents, the design of elaborate and complicated current reference components was required. Unfortunately, the existing solutions to the problem were insufficient. Therefore, there existed a need to provide an improved biasing mechanism for PLLs that avoided the complexity and other problems of the prior art.